I got caught out with a flat battery in my 3.5L petrol Pajero a couple of years ago, and a buddy pulled one of those little jump starters out of her glove box. It's about the size of two hard drives, and I totally thought she was joking, but it actually got my truck started! I'm guessing they work with high voltage and clever regulator to get the current from such a small package, but it really worked. My friend said that the device had already got another vehicle started since it was last charged up, and was doubtful it would start the Paj. I'd have got one myself for those just-in-case moments, but the >$200 p[rice put me off.

BlinkyBill: Well...yeah. But how do I know when it reaches 100%? I mean, this is pretty much why I asked the question in the first place.

This can get a bit technical, but essentially, your battery needs to sit at 14.4 volts (or 14.6 depending on design) until the current draw diminishes to near zero amps. This is fairly technical, which is why "smart' chargers are popular now days - you clip them on, walk away and they 100% charge your battery the shut off so as not to over charging it. The shutoff/startup feature means they're also brilliant for vehicles that don't get driven often - as someone mentioned earlier.

Those 7.2V (nominal) NiMH modules are about 1kg each, consist of 6 x 1.2V cells, and can withstand 100A discharge current. That is enough to start almost any light vehicle.

When those modules fail because of 1 cell out of 6 degrades more - entire module voltage drops below 7V. Those modules have enough "juce" to start your car. So you would need two in series (2kg pack), being compressed (mandatory) and safely connected to the car in the event of jump-start.

I am personally not doing that, but one of my mates told me he and his mate using that method.

I have a seldom-used car with a tiny battery that’s prone to going flat (Honda Jazz) so frequently have to deal with this.

Should just get a small, cheap charge-and-maintain charger and leave it connected to that battery when it's parked up. Lead-acid starting batteries don't like being slow discharged or kept at low voltages.

Also cheap transformer based chargers are at the mercy of line voltage to get enough output to actually charge things all the way, there is a 16% range of allowable voltage on the mains, a dumb transformer one will just pass that 16% fluctuation to the output. Put them on a low line voltage and they struggle to get up to 13v.

Also cheap transformer based chargers are at the mercy of line voltage to get enough output to actually charge things all the way, there is a 16% range of allowable voltage on the mains, a dumb transformer one will just pass that 16% fluctuation to the output. Put them on a low line voltage and they struggle to get up to 13v.

13.7 is a bit low tho, needs 14.4 to 14.6 to get them full. 13.7 will get it to cranking ok stage, but have stuff all reserve capacity in it. Takes days to weeks of typical driving to build that back up again on most cars.